Photoelectrochemistry involves excited-state electron transfer, i.e. light induced oxidation-reduction reactions. The scope of the kinds of reactions induced by light is tremendous. Workers in the fields of organic, inorganic, and physical chemistry and in biochemistry have studied these reactions for synthetic and energy conversion processes. There are abundant opportunities to apply this rich chemistry to bioanalytical problems. This proposal focuses on three of them. 1. Solution-phase photochemistry followed by cathodic reduction of the photosensitizer is the most well-studied system. Advances in instrument ruggedness, detection selectivity and sensitivity will be made. These improvements will bring photoelectrochemistry into the working clinical laboratory. 2. Solution-phase photochemistry yielding a colored product from a colorless reactant will be developed. This chemical system is operationally like having a light-sensitive enzyme. There are significant advantages in stability, selectivity, synthetic flexibility and detectability of the proposed photoelectrochemical label over enzyme labels used in immunoassay. 3. Photoexcitation of a semiconductor electrode surface-bound species followed by electron transfer into the semiconductor electrode will be used to test a novel basis for biosensor development. Fundamental information on the mobility of species attached to a surface through a relatively long organic "tether" will be obtained. This information will be used to develop a prototype antibody-based biosensor.